It occurs naturally in the body in very tiny quantities, but can also be made synthetically.

It has been banned from sports because of its muscle-boosting abilities.

A number of high-profile sportsmen and women have tested positive for the drug, including Linford Christie, but have later been cleared of wrongdoing.

Are tests for nandrolone conclusive?

The by-products of nandrolone can be detected in urine.

If they are found at a concentration greater than two nanograms per millilitre (deemed to be the upper limit at which the substance can occur naturally in the body), the sportsperson may be banned.

But while a test can determine the presence of nandrolone the body, it does not necessarily prove wrongdoing.

More nandrolone could be present if the subject has eaten large quantities of meat contaminated with the substance.

Certain animals may create a bigger risk, particularly horse and boar - and athletes are warned to avoid eating offal from these animals.

It is also possible that dietary supplements which appear perfectly legal can be broken down by the body to produce the same substances created when nandrolone is broken down.

Again, athletes are warned not to believe everything they read on the labels of these supplements.

A UK Sport report on nandrolone said: "We recommend that the sports community should be reminded they must maintain a high level of awareness of the possible hazards of using some nutritional supplements and herbal preparations".

The other source of nandrolone metabolites is other types of steroid - but these are also banned by world sport bodies.

Why do sportspeople take substances like nandrolone?

Anabolic steroids can improve the body's capacity to train and compete at the highest level.

They reduce the fatigue associated with training, and the time required to recover after physical exertion.

They also promote the development of muscle tissue in the body, with an associated increase in strength and power. This is achieved by stimulating the production of protein in the body.

However, some of the increased muscle bulk may be due to the laying down of water and minerals, so the increase in strength may not be as pronounced as expected.

What are the risks associated with anabolic steroids?

Anabolic steroids promote the growth of many tissues in the body by stimulating the release of the hormone testosterone.

By disturbing the body's equilibrium, anabolic steroids can potentially cause damage to many of the body's major organs, particularly the liver, which has to deal with breaking down the compound.

There is also a significant risk of damage to the heart, which is made of muscle tissue. Anabolic steroids can lead to an expansion of the cardiac muscle, which can cause heart attacks.

The drugs also promote the growth of bones, particularly facial bones such as the jaw, and the teeth.

There is also an increased risk of cancer.

Other side effects include:

development of inappropriate sexual characteristics such as breasts in men, and facial hair in women

deepening of the voice

baldness

male impotence

John Brewer, director of the Human Performance Centre at the Lilleshall National Sports Centre, said: "The health risks associated with anabolic steroids are as serious as you can get.

"They greatly increase a person's risk of dying early or of suffering long-term physical problems.

"While the rewards of success in sport are getting greater and greater, the temptation to take anabolic steroids should be offset by the risk of an early grave."